To improve the mechanical properties, workability, and weldability of steel materials, the general practice has been for example to acceleratedly cool a high temperature state steel material right after being hot rolled while processing it on a rolling line and give the steel material a predetermined cooling history. However, the uneven cooling occurring when cooling a steel material becomes a cause of shape defects or work strain in the steel material. Fast improvement is desired to meet with the increasingly tougher demands for better quality of steel materials.
To solve these problems, there is the method of using a plurality of pairs of top and bottom constraining rolls so as to constrain the steel material and prevent heat deformation. However, even with this method, while a steel material with a good shape is obtained, sometimes residual stress inside the steel material manifests itself as deformation at the time the material is worked at the customer side. This is therefore not a fundamental solution. Therefore, uniformly cooling the steel material is the best means for solution.
As a cooling method for achieving uniform cooling, in the method of cooling by using conventional spray nozzles to spray a cooling medium, that is, water, on the steel material, the facilities have been designed so that uniform amounts of water are sprayed in the width direction of the steel material. FIG. 1 shows the nozzle arrangement of a steel material cooling apparatus using conventional plateau shaped water distribution flat sprays. The spray nozzles 1 are arranged in a line at a suitable nozzle pitch S0 in the direction perpendicular to processing so that the distribution of water in the entire region in the direction perpendicular to processing becomes uniform. In the processing direction of the steel material, the adjoining spray regions 2 are arranged so as not to interfere with each other.
However, in a cooling apparatus of this nozzle arrangement, the cooling ability becomes higher at the center of the spray ranges of the nozzles (spray regions 2) compared with the peripheries, so a uniform distribution of cooling ability cannot be obtained in the steel material in the direction perpendicular to processing and uneven cooling sometimes occurs.
As a method of using spray nozzles for uniform cooling, Japanese Patent Publication (A) No. 6-238320 discloses the method of reducing the variation in impact pressure of cooling water in a single spray range to within ±20%. Further, Japanese Patent Publication (A) No. 8-238518 proposes the method of arranging spray nozzles so that spray interference regions are formed. Further, Japanese Patent Publication (A) No. 2004-306064 concludes that uniform cooling can be achieved by having all points in the width direction of a cooled surface pass through coolant spray impact regions at least twice.